(a) Field of the Invention
The present invention relates to a system and method for detecting a misfire occurring in an engine cylinder. More specifically, the present invention relates to a system and method for detecting a misfire occurring in an engine by an electric control unit (ECU).
(b) Description of the Related Art
To prevent air pollution caused by the development of the automotive industry, exhaust gas emissions have been continuously restricted, and accordingly, techniques for electronically controlling engines have been developed. Furthermore, as recent techniques for electronically controlling engines required the adoption of the On-Board Diagnosis 2 (OBD 2) standard that obligates a computer installed in a car to collect information needed to detect malfunctions and to generate alarms, systems and techniques to detect an increase of exhausted gas caused by a misfire and possibility the damage of catalyst are now required. Accordingly, a system and method for detecting an occurrence of a misfire and the engine cylinder at which the misfired occurred, and for restoring the proper operating status of the engine is now desired
In a conventional system and method for detecting an engine misfire, when a piston moves back and forth between a top dead center (TDC) and bottom dead center (BDC) positions in a cylinder and a misfire occurs during a power stroke, a rotational period is measured to detect the occurrence of the misfire since the rotation period of a crank shaft increases because of a reduction of torque of the crank shaft. That is, a variation ratio of the revolutions per minute (rpm) of the engine, or of a signal proportional to the rotation period of the engine detected for each predetermined rotational angle of a crank shaft, is computed using the rotation period of the crank shaft, and when this variation ratio exceeds a predetermined reference value, a misfire is deemed to have occurred. In this event, the variation ratio of the rotation period of the engine is indicated as a difference between a presently measured value and a previously measured value, or a difference of the presently measured value and a moving average value. Since this method represents signals of the rotation period of the engine as a function with respect to time, analysis of the variation ratio becomes difficult during high-speed rotations.
To overcome the above-noted problem, systems and methods to analyze the signals of period of the engine not as a function of time, but as magnitudes and phase angles frequency components of discrete Fourier transformations (DFT) have been proposed.
However, to adopt the above systems and methods, previously computed tabulated values of sine and cosine functions must be set and stored in a memory within an engine controlling apparatus, and routines to perform multiplication operations as many times as the values of the data (i.e., the rpm of the engine), to perform a number of multiplication operations according to the size of the signals, and to determine phases are required. Accordingly, the memory of such systems becomes overused and the time to handle such routines increases.
It is an object of the present invention to provide a system and method for detecting engine misfire while reducing on board computer memory waste and the time to handle calculation routines by using DFT instead of sine and cosine functions.
In one aspect of the present invention, when a misfire occurs in an engine, a frequency analysis is performed on an angular acceleration of the flywheel connected to the crank shaft by DFT, and thereby, the misfire of the engine can be detected by simple addition and subtraction operations.
In more detail, when a misfire occurs in the engine, the angular acceleration that greatly varies because the decrease of output of the engine is detected by a misfire detecting system. In the process of performing a frequency analysis on the angular acceleration of the flywheel, a basis function of the DFT is transformed from a trigonometric function into a Walsh function, and the angular acceleration is sampled between each TDC to find frequency components, and the frequency components are compared with a predetermined reference value so as to determine an occurrence of the misfire.